WordNet (r) 3.0 (2006):

SI unit
    n 1: a complete metric system of units of measurement for
         scientists; fundamental quantities are length (meter) and
         mass (kilogram) and time (second) and electric current
         (ampere) and temperature (kelvin) and amount of matter
         (mole) and luminous intensity (candela); "Today the United
         States is the only country in the world not totally
         committed to the Systeme International d'Unites" [syn:
         Systeme International d'Unites, Systeme International,
         SI system, SI, SI unit, International System of
         Units, International System]